1. Field of the Invention
The present invention relates to a viewing apparatus, which includes (i) a viewing optical system, such as a telescope or binocular, and (ii) a photographing optical system which takes a picture of an object viewed through the viewing optical system, and by which an object being viewed through the viewing optical system can be recorded as a picture image at the same time.
2. Description of the Prior Art
A viewing apparatus, such as a telescope or a binocular, is known as a device for viewing an object at a far distance. However, in such a viewing apparatus of the prior art, the viewed image cannot be recorded as a picture image. There has been the concept of recording a viewed image by combining a viewing apparatus with a camera for a long time. On the other hand, few of such viewing apparatuses has satisfied users, since the viewing apparatus was very large and bulky.
A suitable magnification for a binocular which has been generally used as a viewing apparatus is about 7. At this magnification, an object at a far distance can be viewed in detail without adverse influence on hand shake, while the viewing apparatus is held in user""s hands. However, the field of view which can be obtained by the binocular having a magnification of 7 is equivalent to the field of view obtained through a telephoto lens system with a focal length of 300 mm or more with respect to a 35 mm-format camera. In other words, if an attempt is made to materialize a function of covering the field of view substantially the same as that of a binocular in a general camera/binocular combination, the entire apparatus thereof inevitably becomes larger.
If an attempt is further made to record a picture image with a high quality in the above-mentioned optical arrangement, a higher optical performance is required in the photographing lens system. Consequently, the number of lens elements increases, and cost and weight increases are caused.
Still further, in order to maintain operability as a viewing apparatus, it is desirable for the focusing mechanism of the photographing optical system to move in association with a focusing operation of the binocular. However, since high focusing precision is required in a telephoto lens system having a long focal length of 300 mm as mentioned above, sufficient focusing precision cannot be obtained in an associated movement between the focusing operation of the binocular and the focusing mechanism of the photographing optical system.
The present invention provides a viewing apparatus with a photographing function, while the viewing apparatus is materialized without further enlargement of the apparatus nor a production-cost increase.
As an aspect of the present invention, there is provided a viewing apparatus with a photographing function including a viewing optical system and a photographing optical system. The viewing optical system includes a positive powered (hereinafter, positive) objective optical system and a eyepiece optical system, in this order from the object. An image of an object, formed by the positive objective optical system, is viewed though the eyepiece optical system. The photographing optical system is provided independently from the viewing optical system, and the photographing optical system covers substantially the same field of view as that of the viewing optical system. The viewing apparatus satisfies the following condition:
0.1 less than ft/fo less than 0.7xe2x80x83xe2x80x83(1) 
wherein
ft designates the focal length of the photographing optical system of said viewing apparatus, and
fo designates the focal length of the objective optical system of the viewing optical system.
Preferably, the positive eyepiece optical system of the viewing optical system satisfies the following condition:
5 less than fo/fexe2x80x83xe2x80x83(2) 
wherein
fe designates the focal length of the positive eyepiece optical system of the viewing optical system.
Even when the above conditions (1) and (2) are satisfied (even when the focal length of the photographing optical system in particular is shortened), enlargement of the viewing apparatus can be prevented without sacrificing the recording function of the photographing optical system. This is mainly because of the fact that digital cameras using an image pick-up device (e.g., CCD) have been widely used instead of conventional cameras using a silver-halide film. The CCD has developed rapidly to have higher density. Accordingly, the size of the image plane of a typical CCD is about one tenth of the size of a typical 35 mm-film format.
Moreover, by setting the focal length of the photographing optical system shorter than that of the objective optical system of the viewing optical system, sufficiently high focusing precision can be obtained, even when the focusing mechanism of the photographing optical system is moved in association with the focusing mechanism of the viewing optical system.
The viewing optical system can include a viewing-optical-system focusing mechanism which substantially maintains an image-forming position by moving at least a portion of the viewing optical system in the optical axis direction in accordance with a change in distance to the object.
The photographing optical system can include (i) a photographing-optical-system focusing mechanism which adjusts an image-forming position of an object to a predetermined position by moving at least a portion of the photographing optical system in the optical axis direction in accordance with a change in distance to the object, and (ii) an associated-focusing mechanism which operates the viewing-optical-system focusing mechanism and the photographing-optical-system focusing mechanism in an associated manner.
The viewing optical system includes a positive objective optical system and a positive eyepiece optical system through which an image formed by the objective optical system is viewed. Furthermore, it is practical to use the above viewing optical system as a binocular having a pair of positive objective optical systems and a pair of positive eyepiece optical systems through which an image formed by the objective optical systems is viewed. Still further, it is practical to use the above viewing optical system as a telescope including a positive objective optical system, and a positive eyepiece optical systems through which an image formed by the objective optical system is viewed.
In the case where the viewing optical system is applied to a binocular, the optical axis of said photographing optical system is preferably positioned between the optical axes of the pair of viewing optical systems of the binocular.
The photographing optical system is preferably provided for a digital camera in which an object image is formed on an image pick-up device (CCD), and recorded.
As an example of an optical system used as a telephoto lens system, a Petzval-type optical system is known in the art. A Petzval-type optical system includes a positive first lens group constituted by a positive lens element and a negative powered (hereinafter, negative) lens element, and a positive second lens group constituted by a positive lens element and a negative lens element. A Petzval-type optical system is used as a telephoto lens system since an adequate optical performance is obtained with a relatively smaller number of lens elements.
The photographing optical system of the viewing apparatus preferably includes a positive first lens group constituted by a positive lens element and a negative lens element, and a positive second lens group constituted by a positive lens element and a negative lens element, i.e., a Petzval-type optical system is utilized, and satisfies the following condition:
0.15 less than DI-II/f less than 0.6xe2x80x83xe2x80x83(3) 
wherein
DI-II designates the distance between the positive first lens group and the positive second lens group, and
f designates the focal length of the entire photographing optical system.
The photographing optical system can satisfy the following conditions:
1 less than ACI/ACIIxe2x80x83xe2x80x83(4) 
20 less than "ugr"lpxe2x88x92"ugr"lnxe2x80x83xe2x80x83(5) 
wherein
ACI designates the total sum of the absolute values of the reciprocal of the product of the focal length and the Abbe number (|1/(fi*"ugr"i)|) of each lens element in the positive first lens group; and
ACII designates the total sum of the absolute values of the reciprocal of the product of the focal lengths and the Abbe number (|1/(fi*"ugr"i)|) of each lens elements in the positive second lens group.
"ugr"Ip designates the Abbe number of the positive lens element of the positive first lens group, and
"ugr"In designates the Abbe number of the negative lens element of the positive first lens group.
The positive lens element and the negative lens element of the positive first lens group can be preferably cemented to each other.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-300480 (filed on Sep. 28, 2001) which is expressly incorporated herein in its entirety.